Ventricular tachyarrhythmias are electrical diseases of the heart which may result in "sudden death".
In one type, ventricular tachccardia, the heart muscle, which comprises the ventricles, contracts rapidly in a coordinated fashion. In another type, ventricular fibrillation, which may be a sequela to ventricular tachycardia, there is very rapid and uncoordinated contraction of individual muscles fibers of the ventricles. These rapid heart rhythms result in inefficient, or in the case of ventricular fibrillation, no blood being pumped from the heart and may result in death unless an effective intervention is applied within minutes.
Supraventricular tachyarrhythmias, including atrial fibrillation, atrial flutter, and supraventricular tachycardias, are generally nonlethal arrhythmias that also result in less efficient pumping of blood from the heart, and may result in symptoms of palpitations, pre-syncope and angina.
It is well known in the field of cardiology that these atrial and ventricular tachyarrhythmias can be effectively treated by the application of a sufficiently strong electric shock. Such shocks may be delivered manually by medical personnel via electrodes placed outside the body on the chest wall, or directly on the heart during surgery. Recently, implantable antitachycardia devices have been developed which automatically monitor the heart's rhythm and deliver an electric shock or rapid pacing pulses via implanted electrodes in response to a tachyarrhythmia episode. Likewise, external automatic devices can be used for in and out-of-hospital therapy for ventricular and supraventricular arrhythmias.
Defibrillation output waveforms used by clinically available defibrillators are produced by capacitor discharge. Internal or implantable defibrillators, as well as some external or transthoracic defibrillators, utilize truncated exponential defibrillation waveforms. The waveforms are produced by charging the capacitors to a selected initial voltage and then allowing the capacitors to discharge for a period of time through defibrillation leads placed in or on the body so that current flows through the heart. The rate of capacitor discharge is dependent upon the impedance of the system.
These truncated exponential waveforms can be designed to have either "fixed tilt" or "fixed pulse width". Fixed tilt defibrillators discharge the capacitors from the selected initial voltage until a predetermined final voltage is reached, the "tilt" being the percentage decline in capacitor voltage from its initial value; therefore, the pulse duration varies directly with the system impedance. In contrast, fixed pulse width defibrillators discharge their capacitors for a preselected duration and, as a result, the tilt of the waveform varies inversely with the impedance of the system; low impedances cause the waveform to have a high tilt, while high impedances result in low tilt.
Previous studies (Gold et al., Am Heart J 1979, 98; 207-212; Wessale et al., J Electrocardiology 1980, 13: 359-366; Schuder et al., IEEE Trans Biomed Eng 1983, BME-30: 415-422; Chapman et al., PACE 1988, 11: 1045-1050; Feeser et al., Circulation 1990, 82: 2128-2141) have shown that there is a relationship between the minimum energy or current required for successful defibrillation and the duration of the defibrillation pulse. These experiments demonstrated that the pulse width could be optimized for a given defibrillation waveform and lead configuration. Shorter pulse durations require higher energy to adequately depolarize the myocardium, while longer pulse widths are probably less effective because of their ability to refibrillate the heart.
Some prior art external defibrillators describe adjusting the defibrillation shock based upon impedance (Lerman et al., J Am Cardiol 1988, 12: 1259-1264; Kerber et al., Circulation 1988, 77: 1038-1046). However, these devices do not alter the waveform's pulse duration in response to a previous impedance measurement. The defibrillators delivered damped sinusoidal waveform shocks and either the energy or peak current was adjusted for a transthoracic impedance that was predicted in advance of any shock by passing high frequency alternating current between the defibrillation electrodes. In addition, it is not feasible to use this type of defibrillation waveform or method of predicting inter-electrode impedance in an implantable device.
As explained above, fixed pulse width truncated exponential waveforms will have differing tilts depending upon the impedance of the system. Therefore, the most effective pulse width for a defibrillation waveform will change as the impedance of the system changes. This is particularly true when a biphasic waveform is employed. With a biphasic waveform produced from a single capacitor discharge, the initial voltage of the second negative phase is dependent upon the final voltage remaining on the capacitors at the end of the first phase. If the pulse duration of the first phase is too long for a given system impedance, then the tilt of the first phase will be high, resulting in little voltage remaining on the capacitors and a very low energy and less effective negative phase. If the biphasic waveform's pulse duration-impedance mismatch is large enough, it can result in the delivery of a waveform that is effectively monophasic.
Investigations have shown that, for implantable defibrillator systems, the high voltage lead impedance can change dramatically from that measured at the time of implantation. Typically, the impedance decreases initially, reaching its nadir during the first one to two weeks after implantation, and then gradually increases and stabilizes. In addition, the impedance may change significantly with changes in the patient's clinical course, such as a new myocardial infarction, scarring, worsening or improving heart failure, pericarditis or pericardial effusion. Changes in the defibrillation lead system, including shifting of position, dislodgment or damage, may also cause a large impedance change. These changes in impedance could result in delivery of a preselected fixed pulse width defibrillation waveform which is unable to successfully terminate a tachyrhythmia episode.
It would, therefore, be highly desirable to have available a method of automatically adjusting the pulse duration of a subsequent fixed pulse width truncated exponential defibrillation waveform based upon the impedance measured or calculated following a delivered shock.